Potentially Awesome
This is definately reminicent of Newground's best game, Defend Your Castle. It's not even close to that quality right now, but as an DYC fan knows, DYC started out as a very early BETA. This game is fun right now. You could make it better by first not making it so difficult. Also make more different enemies when you get to higher levels. The Tank is way too powerful as it is right now.
LITTLE KNOWN FACT THAT WILL OFFEND SOME PEOPE: People jump out of the towers when the damage is past a certain point.